A classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics.
The most popular stellar classification system on Terra, thus becoming Earth Starfleet standard (2065 to 2161 CE) and Federation standard (since 2161 CE) later, is described below. It was developed by Annie Jump Cannon of the Harvard College Observatory from 1915 to 1924 CE while she and her colleagues were ordering 225,300 stars into stellar spectra of types O, B, A, F, G, K and M into what was published as the Henry Draper Catalogue shortly after. This classification inspired the mnemonic phrase "Oh Be A Fine Girl/Guy—Kiss Me (Right <Smack> Now)!" still taught to astronomy students today.
The Hertzsprung-Russell diagram (HRD) relates stellar classification with absolute magnitude, luminosity, and photospheric temperature.
|Class||Characteristics||Color||Photospheric Temperature (K)||Examples|
|O||Ionized helium (He II)||blue||28000–50000||Mintaka|
|B||Neutral helium (He I), Balmer series hydrogen||bluish white||9900–28000||Rigel, Spica|
|A||Hydrogen, calcium (Ca II)||white||7400–9900||[[acamar]], Sirius, Vega|
|F||Calcium (Ca II), metals||yellowish white||6000–7400||Canopus, Procyon|
|G||Calcium (Ca II), iron and other metals||yellow||4900–6000||Capella, Sol (G2)|
|K||Large share of metals, later titanium oxide||orange||3500–4900||Arcturus, Aldebaran|
|M||Titanium oxide||reddish orange||2000–3500||Antares, Betelgeuse, Kapteyn's Star|
|R||Cyan (CN), carbon monoxide (CO), carbon||reddish orange||3500–5400|
|S||Zinc oxide||reddish orange||2000–3500|